If the fish is small compared with the wavelength it will be hard to see.
Higher frequencies have shorter wavelengths so can see smaller individual fish.
Higher frequencies are attenuated more so they cannot see as far.
Low frequencies, with long wavelength, can see only whales and big shoals of fish.
Low frequencies can see into the bottom sediment at greater ranges.

A practical constraint is that, to keep a tight beam pattern, the transducer width has to increase in proportion with the wavelength.

Animals like fish typically have swim bladders which cause them to act sort of like big air bubbles. In this case there's some dependency on the resonant frequency of the swim bladder, but in general the sonar returns tend to be much stronger than you'd otherwise expect.